Fog
The unrevealed part of the map hiding enemy heroes outside of vision range.
Fog (short for fog of war) is any part of the map not currently revealed by an ally hero, ward, or vision item — enemies standing in fog are completely hidden regardless of proximity. Bushes function as permanent fog even at close range unless a hero has bush-detection vision, which is why junglers and assassins path through them to approach a gank unseen. Managing fog is a core map-awareness skill separating safe laners from ones who get repeatedly ambushed.